A printed circuit board blank is a laminate that comprises a layer of electrically conductive material such as a copper plate or the like and a layer of photographically light sensitive material such as an ultra-violet sensitive film which is commonly referred to as "dry film resist" on a core of material which typically is comprised of fiberglass resin mesh.
Typically, the printed circuit board blank may be square or rectangular. However, it can be any suitable shape.
The layer of photographically light-sensitive material is laminated to the layer of electrically conductive material in a suitable laminating machine. Typically, the laminating machine brings the layer of photographically light-sensitive material into touching relation with the surface of the copper plate to form an assembly. Then, heat and pressure are applied by conveying the assembly through a set of opposed rollers. The rollers apply heat and pressure to the layer of photographically light-sensitive material and the layer of electrically conductive material so that the layer of photographically light-sensitive material is bonded to the layer of electrically conductive material to form the laminated printed circuit board blank.
Typically, the rollers in each set of rollers are movable toward and away from each other. This is so that when, for example, the layer of electrically conductive material is a copper plate, its leading edge and/or trailing edges which may be rough or unfinished do not damage the surface of the rollers. Thus, damage is avoided by not bringing the rollers in each set together until the leading edge of the copper plate is between them and then retaining them together until just as the trailing edge is exiting.
In this regard, each of the rollers may be comprised of heavy duty solid stainless steel. The rollers may be coated with rubber or other suitable material having a durometer hardness of about 70 on the Shore A scale. While for many applications 70 on the Shore A scale is satisfactory, is it is apparent that the hardness of the roller surface or the scale by which it is measured do not form part of the invention, but are included to provide a more complete description of the prior art.
After the layer of photographically light-sensitive material is laminated to the layer of electrically conductive material to form the printed circuit board blank, the circuit which is to be formed on the printed circuit board blank is optically projected onto the layer of photographically light-sensitive material in a manner similar that used in a conventional still camera where the image is projected onto the light sensitive film.
However, in contrast to a conventional camera the film used in the manufacture of printed circuit boards is sensitive to ultra-violet light rather then visible light. This enables it be handled in a factory environment with out fear of inadvertently exposing it. The exposed photographically light sensitive material is removed when the photographically light sensitive material is "developed" leaving the desired portions of the surface of the copper plate exposed. The desired circuit is then etched into the layer of electrically conductive material in the places where the photographically light sensitive material has been removed.
It is essential in the manufacture of the printed circuit board blanks that there be a complete bond between the photographically light sensitive material and the surface of the electrically conductive material. If the photographically light sensitive material is not completely bonded to the surface of the electrically conductive material, the circuit which is projected onto the photographically light sensitive material may not be in sharp focus resulting in a distorted circuit being etched into the electrically conductive material. Further, during etching, the etching fluid can flow under the layer of photographically light sensitive material if it is not bonded to the surface of the electrically conductive material which could result in other inaccuracies, short circuits or the like.
Even when great care and attention is paid to the manufacturing process, a substantial portion of printed circuit board blanks are defective as a result of incomplete lamination of the photographically light sensitive material and electrically conductive material. Typically, the defects include air bubbles, scratches or other surface defects which prevent the layer of photographically light sensitive material from completely bonding to the surface of the electrically conductive material, or the bond fails to occur at the edges of the electrically conductive material.
This may result from a defect in the laminating process, an unevenness in the surface of the electrically conductive material on which the bond is to be made, unevenness in the photographically light sensitive material, or the like.
Consequently, up to forty percent of printed circuit board blanks are unusable.